Fuel 121 (2014) 65–71

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Formation of thiophenic species in FCC gasoline from H2S generatingsulfur sources in FCC conditions

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.12.033

⇑ Tel.: +55 21 216 26675; fax: +55 21 216 26626.E-mail address: gilbertw@petrobras.com.br

William Richard Gilbert ⇑PETROBRAS S.A./CENPES, Research & Development Centre, Av. Horacio Macedo, 950, Cidade Universitaria, Quadra 7, 21941-915 Rio de Janeiro-RJ, Brazil

h i g h l i g h t s

� H2S, olefins recombination reactionsto mercaptans and thiophenes in FCClab scale reactor.� Cracking of dymethyl-dysulfide

(DMDS) doped sulfur free diesel in anFCC lab scale reactor.� Cracking DMDS doped diesel with

zinc oxide based gasoline sulfurreduction additives.

g r a p h i c a l a b s t r a c t

Olefin-H2S recombination to form mercaptans and thiophenes was simulated in the lab by cracking ultra-low sulfur diesel spiked with dymethyl-dysulfide (DMDS) as a proposed model reaction for FCC sulfurchemistry in a micro-reactor system producing gasoline range thiophenes. When the experiment wasrepeated in the presence of ZnO containing catalyst thiophene formation was inhibited by zinc, whichacted as an H2S trap.

a r t i c l e i n f o

Article history:Received 8 October 2013Received in revised form 5 December 2013Accepted 14 December 2013Available online 26 December 2013

Keywords:ThiopheneFCC catalystH2S recombinationSulfur in gasoline

a b s t r a c t

Sulfur contamination of a bio-gasoline produced from very low sulfur soybean oil in a circulating pilotriser led to the investigation of the H2S-olefin recombination pathway by which any sulfur source capableof producing H2S in the FCC riser could in principle produce all of the typical thiophenic species present inthe FCC cracked naphtha. The H2S-olefin recombination mechanism was confirmed in the laboratory bycracking ultra-low sulfur diesel spiked with 2 wt% dymethyl-dysulfide (DMDS) in a Short Contact TimeResid Test reactor to produce 60–80 ppm gasoline, in which the typical cracked naphtha sulfur specieswere major components. DMDS cracked to produce methyl-mercaptan and H2S as primary products,which continued to react with hydrocarbons derived from the diesel oil cracking to produce the interme-diates and final products in the reaction pathway. When the reaction was repeated with catalyst contain-ing 10% of a commercial gasoline sulfur reduction additive, the sulfur in gasoline was reduced by 40% andthe sulfur in the spent catalyst went from very low levels, when no additive was used, to 0.5 wt% with theadditive. The importance of the recombination pathway and the catalyst additive activity in interferingwith the pathway by acting as an H2S scavenger may explain the functional mechanism of ZnO based gas-oline sulfur reduction additives.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Fluid Catalytic Cracking (FCC) is one of the chief conversion pro-cesses in a petroleum refinery, producing cracked naphtha, distil-late and C3, C4 streams from vacuum gasoil and other heavyfeedstocks. Cracked naphtha is often the principal component ingasoline formulation and the most important source of sulfur inthe pool. Depending on the crude oil of origin, cracked naphthamay have a relatively high sulfur concentration, varying from2000 mg/kg to 200 mg/kg, well above the maximum sulfur limits

of automotive gasoline defined by legislation in several countries,which is typically below 50 mg/kg [1]. The gasoline sulfur reduc-tion process of choice is the hydrodesulfurization of the crackednaphtha, however, depending on the refinery configuration, otheralternatives such as reducing the cracked naphtha distillationend point or using gasoline sulfur reduction catalysts in FCC mayalso be applied. Additionally, naphtha hydrodesulfurization willsaturate olefins and reduce the research octane number, and inmany instances it is interesting to use a combination of differentstrategies to achieve a given sulfur limit [2].

For the reasons discussed above, understanding the sulfurchemistry in FCC has been important in the industry and many lab-oratories have worked on the problem. Chemically stable

66 W.R. Gilbert / Fuel 121 (2014) 65–71

thiophene derivatives are the dominant sulfur species in the FCCgasoline. In a typical cracked naphtha from a Brazilian refinerywith 800 mg/kg sulfur, 10% of the sulfur was thiophene, 25%C1-thiophene isomers, 30% C2-thiophenes, 10% C3-thiophenes,10% benzothiophenes and 15% C1-benzothiophenes; whereasother sulfur compounds, such as C3+ mercaptans, dysulfides andtetrahydrothiophenes appear in minute amounts. The sulfur distri-bution in the gasoline boiling range shows a sharp increase in con-centration towards the end of the curve because of the presence ofbenzothiophenes, which boil above 220 �C and are concentrated inthis fraction. The prevalent mechanism for the production of sulfurcompounds in the FCC liquid product in the literature [3] proposesa network of reactions in which heavy sulfur compounds in thefeed crack to directly produce the thiophenic species observed inthe products; a secondary route to thiophenes is also proposed,in the same network, starting from intermediate mercaptanswhich are produced by cracking of the feed or from H2S-olefinrecombination reactions (Fig. 1). The relative contribution of theprimary and secondary routes is not adequately described in theliterature as most of the experimental work used either native ga-soils as FCC feeds or blends in which small amounts of thiopheniccompounds were added to the main feed [3–7]. Experiments thatspecifically probed the mercaptan/olefin-H2S to thiophene path-way [8,9] showed a relatively low conversion of the starting mate-rials to thiophenes suggesting the subsidiary nature of this route.

FCC gasoline sulfur reduction catalyst additives are described inthe literature using ZnO as active ingredient [7,8,10,11]. Severalsulfur reduction mechanisms are proposed, most of which willemphasize the Lewis acid character of the active ingredient andits capacity to either facilitate hydrogen transfer to alkyl-thio-phenes to produce tetrahydrothiophene [12] or adsorb thiopheniccompounds to produce coke [13]. Additive effectiveness in com-mercial applications is limited, typically achieving a maximumgasoline sulfur reduction of 25% or less, depending on several fac-tors, including aromatic character of the feed, base catalyst formu-lation and metal contamination [5,14]. Transition metals used asactive ingredients in the additive will tend to promote hydrogentransfer reactions in FCC conditions. Hydrogen transfer reactionsare beneficial to the sulfur reduction strategy, as saturation ofthe thiophenic ring is required before hydrodesulfurization can oc-cur. However, hydrogen transfer reactions are detrimental to theFCC catalyst selectivity, producing extra coke and hydrogen whichaggravate air blower and wet gas compressor constraints and mayimpose a limit to the amount of gasoline sulfur reduction additivewhich can be used.

This work describes an attempt to produce low sulfur bio-gaso-line from soybean oil cracking which resulted in the production ofgasoline thiophenic sulfur species from a virtually sulfur free feed.

Fig. 1. FCC sulfur chemistry mechanism showing primary reactions (1) whichproduce olefins, H2S, mercaptans (R-SH), thiophenes (R-T), benzothiophenes(R-BzT), dibenzothiophenes (R-dBzT) and coke. Secondary reactions are also shown:(2) H2S-olefin reversible recombination to produce mercaptans; (3) mercaptancyclization to produce tetrahydrothiophene (R-THT); (4) dehydrogenation toproduce thiophene from tetrahydrothiophene; and (5) condensation reactionswhich produce heavier sulfur derivatives from lighter precursors.

The possible sulfur contaminants identified at the time that couldgenerate the observed gasoline sulfur would require the formationof the gasoline thiophenic species starting from a generic sulfursource that would first be converted to H2S and than to thegasoline range sulfur species by combining with intermediatehydrocarbons produced in the triglyceride cracking. Follow upexperiments in the laboratory produced results which demon-strated that the H2S-olefins to thiophenes mechanism was capableof producing the gasoline sulfur species in the concentrations ob-served in the soybean oil cracking experiment. This mechanismis probably more important than previously thought in conven-tional FCC and presents an alternative explanation to the modeof action of ZnO based gasoline sulfur reduction additives.

2. Experimental

2.1. Soy bean cracking in the FCC pilot riser

The Petrobras FCC pilot riser located in the SIX refinery in SaoMateus do Sul, Parana, Brazil, is a 200 kg/h feed rate circulatingunit with an 18 m riser and 400 kg catalyst inventory. Fig. 2 showsthe schematics of the reactor, regenerator and product recoverysystem. The unit has an adiabatic reactor, stripper and regeneratorfor heat balance studies. The soybean oil used in the experimentwas food grade degummed oil supplied by Incopa LTDA with0.9193 specific gravity and 30 mg/kg sulfur content. Because thevery low coke yield produced by soy bean oil cracking is insuffi-cient to meet the FCC heat requirement, 1800 mg/kg sulfur,43,100 kJ/g HHV diesel oil was burned in the regenerator as torchoil. Prior to product collection, the unit was run for a few hours un-til stable operation in the specified conditions was achieved andthan run for 1 h to wash out the product recovery system withthe product generated at the specified conditions, at this pointproduct collection would be performed for another 1 h period.The feed, regenerator air, flue gas and gas product flow rates weremeasured with Coriolis mass flow meters. Flue gas and product gascompositions were analyzed by gas chromatography and the liquid

Fig. 2. Pilot riser reactor schematic diagram.

Table 1FCC pilot riser unit operating conditions.

Feed rate(kg/h)

Riser T (�C) Regen. T (�C) Feed preheatT (�C)

Torch oil(kg/h)

200 530 690 80 3.2

Fig. 3. SCT-RT reactor schematic diagram.

Table 2SCT-RT test series.

Conditions A B C D

Feed sulfur 5 mg/kg 1.86% 1.72% 1.72%Sulfur source Non CS2 DMDS DMDSCatalyst Ecat 1 Ecat 1 Ecat 1 Ecat 2

W.R. Gilbert / Fuel 121 (2014) 65–71 67

oil product and the water condensate were collected and weighed.The coke yield was calculated from the flue gas mass flowrate andcomposition. Gasoline, LCO and slurry yields were calculated fromthe liquid product simulated distillation (ASTM D 2887) using221 �C and 343 �C as cut points. Gasoline octane was calculatedfrom the detailed hydrocarbon analysis chromatography of thegasoline fraction. A preliminary run was performed to check onthe gasoline quality before the final run was done. An equilibriumcatalyst from one of Petrobras refineries was used in the tests (PRUEcat). The FCC operating conditions used in the run are specified in

Table 3SCT-RT feed – hydrotreated diesel oil properties.

Spec. gravity 20/4 �C Sulfur (mg/kg) Sim. distill. IBP (�C) Sim. distill. 10 wt% (

0.8241 4.8 103 152

Table 4Catalyst chemical properties.

Catalyst Al2O3 (%) SiO2 (%) RE2O3 (%) Na2O (%) P2O5 (%)

PRU Ecat 43.2 52.2 2.59 0.31 0.75SCTRT Ecat 1 48.4 46.1 2.93 0.38 0.62SCTRT Ecat 2 47.3 42.6 2.65 0.53 0.58

Table 1. The liquid product from the soy bean oil cracking was latercollected and distilled in a TBP distillation column, generatingshort cut fractions which were then characterized for sulfurcontent.

2.2. Short Contact Time Resid Test experiments

The idea behind the laboratory scale tests was to emulate theresults from the pilot unit, by running a very low sulfurhydrotreated diesel with an external source of sulfur which wouldbe converted to H2S in the FCC reaction conditions. A SCT-RT unit[15] was chosen rather than a Fixed Fluidized Bed (ACE) unit[16], after preliminary tests with CS2 in the ACE unit showed verylittle conversion of the CS2. Fig. 3 shows a schematic diagram ofthe SCT-RT reactor. In the SCT-RT the feed is injected into the veryhot catalyst bed (670 �C), similar to what happens in circulatingunits, thus accelerating initiation reactions. Nevertheless, the at-tempt to convert CS2 in the SCT-RT reactor also proved unsuccess-ful so that the sulfur source was changed for dymethyl-dysulfide(DMDS), which worked as expected. Table 2 shows the feed/cata-lyst combinations that were evaluated in the test series. SCT-RTreactor catalyst temperature was 670 �C in all runs and catalyst/oil ratio was varied between 5 and 7 for each catalyst-feed pair,feed injection time was 1 s, feed injection rate was 2 g/s. Gas prod-uct was analyzed from the sample collected from the water dis-placement vessel, resulting in a wide range of variation of H2Sconcentration in the gas product, because of partial solubilizationof H2S in the water. Liquid product yields, gasoline (C5-216 �C),LCO (216–343 �C) and bottoms (343 �C+) were calculated usingGC-Simulated Distillation based on ASTM D 2887. Mass balanceswere within 95% to 100% range. Sulfur analysis of the feeds wasmeasured by an Antek 7090 Chemiluminescence analyzer (ASTMD 5453); sulfur analysis of the liquid product was done by Gaschromatography based on the ASTM D 5623 standard. Propertiesof the hydrotreated diesel used in the feed blends are shown in Ta-ble 3. DMDS and CS2 were 99% grade reagents supplied by Aldrich.Catalyst analysis by X ray fluorescence is shown in Table 4. TheEcat evaluated in condition D used zinc containing about 10% com-mercial gasoline sulfur reduction additive.

3. Results and discussion

3.1. Soy bean oil cracking results

Soy bean oil is a mixture of polyunsaturated triglycerides con-taining linoleic, oleic, stearic and palmitic acids with C57H100O6average chemical formula. The fatty acid hydrocarbon chains ofthe molecule readily crack to produce a product profile similar tothat of a paraffinic hydrocarbon feed. The carbonyls and the hydro-gen deficient glycerol portion of the molecule will crack to producemostly coke, gas, CO, CO2 and water. Conditions in the pilot riser

�C) Sim. distill. 50 wt% (�C) Sim. distill. 90 wt% (�C) Sim. distill. FBP (�C)

228 329 446

Fe2O3 (%) TiO2 (%) MgO (%) ZnO (%) Ni (mg/kg) V (mg/kg)

0.43 0.23 nd nd 1053 5440.54 0.36 0.03 0.01 1240 19200.49 0.33 4.11 0.80 1100 1730

0

30

60

90

120

150

180

0 50 100 150 200 250TBP °C

Sulfu

r mg/

kg

Fig. 4. Sulfur concentration profile as a function of the boiling temperature of thesoybean oil cracked product. Caption: + Run 1, 4 Run 2.

68 W.R. Gilbert / Fuel 121 (2014) 65–71

were chosen so as to avoid gasoline over-cracking and meet gaso-line specifications of minimum sulfur content (preferably less than10 ppm) and highest octane possible. Table 5 presents the gasolineyield and selected properties, the torch oil consumption and theregenerator air rate of the three runs and Table 6 the carbon,hydrogen and oxygen balances for run number 1, based on averagehydrocarbon product C/H ratios. Hydrogen recovery in the gasolinefraction was high, showing a satisfactory balance between hydro-gen lost to the LPG (conversion too high) and LCO fractions (con-version too low). Oxygen recovery was less than 100%, anindication of the presence of oxygenated products in the oil andwater fractions. The existence of oxygenates in the oil fractionwas later confirmed by the detection of 2% of propanaldehydeand smaller amounts of other aldehydes, ketones and phenols,adding up to another 0.8% of the bio-gasoline product.

Table 5 shows that the gasoline sulfur of run 1 was substantiallyhigher than the 10 ppm target. This result came as a surprise be-cause of the very low sulfur level in the soy bean oil feed. Liquidproduct contamination was unlikely because of the large volumesprocessed in the pilot riser and the routine product recovery sys-tem conditioning precautions. For instance a nearly 10% contami-nation with the typical 600 ppm Brazilian fossil VGO crackednaphtha produced in the unit would have been necessary to

Table 5Soy bean oil pilot riser cracking results – yields and gasoline quality as a function of oper

Run Riser T (�C) Regen. T (�C) Torch oil (kg/h) Regen. air rate (kg/h) Mas

1 530 690 3.6 168 1012 500 701 2.2 156 99

Table 6Soy bean oil pilot riser cracking results.

Yields C/H

Mass balance wt% 101.8 –Dry gas (w/o oxygen spc) wt% 1.7 0.27LPG wt% 9.4 0.46Gasoline (C5–221 �C) wt% 46.6 0.58LCO (221–343 �C) wt% 20.1 0.86Bottoms (343 �C+) wt% 5.5 0.90Coke wt% 3.8 1.00CO wt% 1.6 –CO2 wt% 0.5 –H2O wt% 10.3 –

Table 7TBP distillation results of the soy bean bio-gasoline.

Run 1

Cut points (�C) Cumulative wt% of gasoline S in fraction (mg/kg)

IBP – 34 �C 6.0 nd34–62 �C 12.4 262–68 �C 18.7 268–82 �C 25.4 382–92 �C 31.8 492–104 �C 38.5 8104–112 �C 45.5 16112–130 �C 52.4 21130–138 �C 59.5 26138–156 �C 66.8 29156–168 �C 74.3 48168–182 �C 81.9 97182–198 �C 89.6 153198–221 �C 100.0 157

221–FBP �C – 142

explain the 50 ppm sulfur in the soy bean gasoline. The TBP col-umn distillation of the bio-gasoline product from run 1 and thecharacterization of the fractions (Table 7) generated the sulfurconcentration profile displayed in Fig. 4, in which the inflectionpoints are strongly suggestive of the presence of thiophenes(BP P 100 �C) and benzothiophenes (BP P 190 �C). With no clearsulfur source to explain the soy bean cracked naphtha sulfur

ating conditions.

s balance (wt%) Gasoline (wt%) Gasoline GC MON Gasoline sulfur (ppm)

.8 46.6 77 50

.1 40.6 75 11

Carbon % Hydrogen % Oxygen %

100.0 99.9 95.61.7 3.5 0.0

10.2 12.7 0.052.4 51.5 0.023.6 15.6 0.0

6.5 4.1 0.04.5 2.3 0.00.9 0.0 8.40.2 0.0 3.30.0 10.1 83.9

Run 2

Cut Points (�C) Cumulative wt% of gasoline S in fraction (mg/kg)

IBP-42 �C 7.1 nd42–71 �C 17.1 171–93 �C 27.4 293–111 �C 37.9 3111–131 �C 48.6 5131–149 �C 59.8 9149–171 �C 71.0 10171–189 �C 82.7 13189–209 �C 94.6 26209–221 �C 100.0 42

221-FBP �C – 79

Table 8Sulfur mass balance in the soybean oil cracking experiment.*

Feed Torch Inputs Gasoline 211 �C+ Liq. product

kg/h 200 3.6 93.2 51.2mg/kg 30 1800 50 142mg/h 6000 6480 12,480 4660 7270 11,930

* H2S in the gas product and SO2 in the regenerator flue gas were not measured.

W.R. Gilbert / Fuel 121 (2014) 65–71 69

products, suspicion fell on the 1800 ppm sulfur diesel used fortorch oil. For the sulfur in the torch oil burned in the regeneratorto be converted into fuel sulfur it would first have to be trans-ported as SO2 as a gas or adsorbed on the catalyst from the regen-erator to the riser, reduced to H2S and converted to thiophenederivatives via a olefin-H2S recombination pathway. Reactortemperature and regenerator temperature were changed to reducetorch-oil consumption and catalyst circulation, thereby reducingflue gas sulfur carryover to the riser, which was believed to beresponsible for the high sulfur gasoline. The liquid product distilla-tion and characterization at the end of run 2 showed that the lowsulfur soy bean cracked naphtha was successfully produced.

The sulfur mass balance which was later performed (Table 8),required very efficient transport of sulfur from the regenerator tothe reactor and a very high conversion of the total sulfur input tothe liquid product sulfur for the proposed mechanism of gasolinesulfur species generation in the soybean cracking experiment tobe valid. Any of the two events are very unlikely to have occurred.The amount of flue gas passively carried over with the catalystfrom the regenerator to the reactor was measured as 0.4%, basedon the inert gas concentration (N2 and CO2) in the reactor productgas from another group of experiments in the same unit with fossilVGO feedstock. SOx additives are capable of actively transporting

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

74 76 78 80 82 84

Conversion (<221°C) wt%

Dry

Gas

wt%

(A)

50.0

52.5

55.0

57.5

60.0

74 76 78 80 82 84

Conversion (<221°C) wt%

Gas

olin

e (C

5-22

1°C

) w

t%

(C)

Fig. 5. SCT-RT yields: (A) dry gas, (B) methane, (C) gasoline and (D) coke as a function of(�) diesel plus DMDS condition ‘‘C’’, (}) diesel plus DMDS with sulfur reduction catalys

sulfur oxides from the regenerator to the reactor, however, therewere no such additives in the catalyst which was used in the soy-bean oil cracking.

Even though the change in operating conditions to produce thelow sulfur gasoline was successful, the reason for the gasolinesulfur reduction was probably not related to the reduction in sulfurtransport from the regenerator to the riser and the ultimate sulfurcontamination cause remained unexplained.

3.2. SCT-RT cracking results

Fig. 5 shows the yield profiles obtained in the SCT-RT crackingexperiments. The high conversion and low coke yield obtained inthe SCT-RT diesel cracking experiments was already expecteddue to the light paraffinic nature of the feed. Adding the sulfur dop-ing components CS2 in condition B and DMDS in conditions C and Dhad little noticeable effect on the product yields, except for the in-crease in dry gas yield which was explained by the increase inmethane production, a residue from the cracking of the sulfur dop-ing species (Fig. 5A and B).

Regarding the sulfur speciation of the SCT-RT products, none ofthe gasoline sulfur species were detected in the pure diesel crack-ing runs (condition A). Table 9 shows the results from selected runsfrom conditions B, C and D, in which a sulfur source was blendedwith the ultra low sulfur diesel. The sulfur compounds used inthe doping experiments introduced a new degree of complexityto the sulfur chemistry mechanism. Both CS2 and DMDS are nucle-ophiles in their own right and will interact with the FCC catalyst.CS2, used in condition B, was relatively unreactive and much of itwas recovered unscathed (off scale) in the cracked oil productresulting in the high liquid product total sulfur concentrationsmeasured in condition B shown in Table 9. DMDS, however, readily

0.0

0.4

0.8

1.2

1.6

72 74 76 78 80 82 84

Conversion (<221°C) wt%

C1

wt%

(B)

0.0

0.5

1.0

1.5

2.0

2.5

74 76 78 80 82 84

Conversion (<221°C) wt%

Cok

e w

t%

(D)

conversion. Caption: (+) pure diesel condition ‘‘A’’, ( ) diesel plus CS2 condition ’’B’’,t condition ‘‘D’’.

Table 9Sulfur speciation of SCT-RT cracking products.

Condition B B C C D DSulfur source CS2 CS2 DMDS DMDS DMDS DMDS

Catalyst Ecat 1 Ecat 1 Ecat 1 Ecat 1 Ecat 2 Ecat 2Prod. total sulfur mg/kg 2000 3000 230 187 118 112CTO 7.0 6.5 5.0 6.0 6.0 6.0

H2S mg/kg 0.06 0.05 0.16 0.17 0.11 0.05COS mg/kg 0.14 0.23 – – 0.13 0.05C1-SH mg/kg 0.25 – 1.40 6.37 1.02 1.46C2-SH mg/kg 0.15 – 1.35 2.56 0.80 0.36DMS mg/kg* – – 31.99 49.40 14.64 13.40CS2 mg/kg* off scale off scale 2.83 0.00 0.75 0.55C3-SH mg/kg 0.09 – 3.59 4.67 1.84 1.39C4-SH mg/kg – – 0.78 0.72 0.90 0.72E-M-sulfide mg/kg* – – 11.25 12.24 6.51 4.02Thiophene mg/kg 4.64 3.45 7.71 8.28 2.44 2.76DE-sulfide mg/kg* – – 1.24 0.97 0.36 0.18M-P-sulfide mg/kg* 0.06 – 6.35 6.73 3.69 2.09DMDS mg/kg* – – 0.33 0.45 3.94 5.17C5-SH mg/kg – – 2.92 3.01 1.23 0.28M-thiophene mg/kg 3.62 3.19 15.06 13.78 6.73 6.90DM-thiophene mg/kg 3.24 3.19 11.04 9.76 5.14 5.05TM-thiophene mg/kg 2.43 2.38 12.76 13.20 11.01 19.41TH-thiophene mg/kg 1.25 0.31 3.00 3.16 1.00 1.37Benzothiophene mg/kg 0.62 0.68 0.93 1.04 1.00 1.00M-benzothioph. mg/kg 2.35 2.29 5.30 6.74 4.05 3.45

S in spent catalyst wt% nd 0.07 0.028 0.041 0.42 0.44C3, C4-SH + Thioph. mg/kg� 18.24 15.49 63.09 64.34 35.33 42.32

* Products resulting from the sulfur doping source not present in significant amounts in typical FCC cracked oil.� Products representing total sulfur in gasoline concentration.

70 W.R. Gilbert / Fuel 121 (2014) 65–71

reacted not only producing H2S, as intended, but also a series of dif-ferent thioethers formed by chain reactions analogous to those in-volved in hydrocarbon Bronsted acid mediated cracking [17](Fig. 6). The total liquid product sulfur concentration in conditionC (DMDS runs) was much less than in conditions B because mostof the sulfur originally in the feed was lost as H2S to the gasproduct.

Although thiophene and its derivates were not present in thefeed, they were detected in significant amounts in the SCT-RTproduct in all cases in which a sulfur source was provided, demon-strating the importance of the mercaptan and olefins-H2S recombi-nation pathway. Because of its reactivity, DMDS was much moreefficient than CS2 in transferring the sulfur to the gasoline fractionof the product, resulting in sulfur content in gasoline greater than60 ppm at condition C. The heavier aromatic thiophene derivates,benzothiophenes and alkyl-benzothiophenes, were detected inmuch lower concentrations than in VGO FCC products, possibly be-cause there was not enough time for the multi-step condensationreactions or because of the relatively light molecular weight hydro-carbon feed. The SCT-RT results in this sense contrasted with thesoy bean oil cracking results, which showed a benzothiophene tothiophene ratio more typical of conventional FCC. When the zinccontaining additive catalyst (Ecat 2) was used (condition D) thesulfur content in the liquid product was reduced by nearly 40%.The lower sulfur in gasoline for Ecat 2 was compensated by a more

Fig. 6. Illustrative example of a Bronsted acid mediated chain reaction mechanismproducing different thioethers either by the transfer of fragments from hydrocarbonmolecules present in the reaction mixture or by the reaction of a mercaptan to anadsorbed carbocation.

than ten times increase in the sulfur content of the spent catalyst(Table 9), an indication that the additive was acting as a sulfur trapand inhibiting the pathway leading to gasoline fraction sulfurproducts.

The activity of the zinc oxide as a sulfur trap, immobilizing thesulfur as a stable zinc sulfide and inhibiting one group of manypossible reaction pathways to gasoline sulfur species in FCC ismore plausible than other hypotheses discussed in the literature,some of which would require the very unlikely event of saturationof the aromatic thiophenic ring in a low H2 partial pressure andsubsequent desulfurization. By hindering only one of the possiblesulfur chemistry pathways, the gasoline sulfur reduction additivemay prove to be inherently incapable of achieving the very highgasoline sulfur reductions required for compliance with the mod-ern fuel specifications.

4. Conclusions

A sulfur source capable of being converted to H2S in FCC reac-tion conditions can produce any of the naphtha range thiophenicspecies common in FCC gasoline in substantial concentrations byH2S-olefin recombination to form mercaptans followed by mercap-tan cyclization and dehydrogenation. This mechanism has alreadybeen described in the literature; however its relative importancehas been overlooked.

The DMDS experiments successfully demonstrated the H2S/mercaptan to thiophene formation in FCC conditions and may beused as a model reaction for the development of FCC gasoline sul-fur reducing additives. The production of a range of sulfur deri-vates, such as thioethers, starting from DMDS, although notdirectly related to FCC chemistry, is an interesting example of theinteraction of the FCC catalyst with a non hydrocarbon substratein a reaction pathway, which has similarities to the reaction mech-anisms proposed for hydrocarbon cracking.

The experiments with zinc oxide containing catalyst resultingin a large reduction in thiophene production and a large increase

W.R. Gilbert / Fuel 121 (2014) 65–71 71

in the sulfur content in spent catalyst strongly suggest that the zincbased additive would be acting as sulfur sorbent, removing H2Sfrom the reaction media and inhibiting the formation of gasolinesulfur species via the olefin-H2S recombination pathway.

Acknowledgments

The author would like to thank Cleber Ursini for his help withthe sulfur speciation analysis and Petrobras for permission to pub-lish this work.

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